1. Field of the Invention
The present invention relates to photochromic photosensitive compositions and, more specifically, it relates to photochromic photosensitive compositions having additives containing at least two aromatic rings to improve the storage stability of the material in its colored state.
2. Description of the Prior Art
Compositions containing spiropyran compounds as photosensitive materials develop colors upon irradiation with ultraviolet rays to perform recording, and regain their initial colorless state upon heating or irradiation by visible rays. In the recording materials used in such a chemical process, measures have been taken for improving the heat stability of colored spiropyran compounds in order to preserve the recordings over a longer period of time.
For example, it has been suggested to add simple phenols to a composition containing a indolinobenzopyran-based spiropyran compound having a color developing wavelength at a shorter wavelength region (below 600 nm) as a photosensitive material, thereby improving the heat stability of the heat sensitive material in its colored state. There has further been proposed a indolinobenzothiopyran-based spiropyran compound which has superior heat stability properties in its colored state, to the indolinobenzopyran-based spiropyran compound described above, and which shows high absorption characteristics at or above the coloring wavelength of 700 nm, particularly, near 780 nm which is the oscillation wavelength region for a semiconductor laser.
However, there has been a problem in that the indolinobenzopyran-based spiropyran compound evidences a lowering of color density and the color wavelength band shifts toward the shorter wavelength side. In addition, the simple phenols bleed to the surface of the composition since they are less compatible with high molecular weight binders.
On the other hand, in the case of the indolinobenzothiopyran-based spiropyran compound, simple phenols are not as effective where it is intended to improve the stability of the compound in its colored state and preserve the recordings over a longer period of time.